Electrically driven implantable devices are used principally as cardiac pacemakers, but they have also been considered for defibrillators; for heart assist, or drug infusion and dispensing systems; for bone growth and repair, pain suppression, or scoliosis treatment; for artificial vision, heart,or larynx; for stimulation of brain, nerves, muscle, gut or bladder; and for implanted sensors. For purposes of this disclosure, an implantable cardiac pacemaker/defibrillator is given as an example.
The basic pacemaker system consists of an electrode attached to the heart and connected by a flexible lead to a pulse generator. This generator is a combination of a power source and the microelectronics required for the pacemaker system to perform its intended function. A fixed rate pacemaker provides continuous pulses to the heart, irrespective of proper heart beating, while a demand inhibited pacemaker provides pulses only when the heart fails to deliver a natural pulse. Depending upon various sensed events, the pacemaker stimulates the right atrium, the right ventricle, or both chambers of the heart in succession. The pacemakers in current use incorporate circuits and antennae to communicate non-invasively with external instruments called programmers. Most of today's pacemakers are of the demand inhibited type, hermetically sealed, and programmable.
The longevity of pacemakers has been limited primarily by the capacity of their power sources. Early pacemakers were powered by primary zinc-mercuric oxide cells. Although this system was used for about 15 years, it did suffer from high self-discharge and hydrogen gas evolution. Because of gas evolution, the pacemaker could not be hermetically sealed, and had to be encapsulated in heavy epoxy. In 1970, the average life of the pulse generator was only 2 years, and 80 percent of explants were necessitated by failed batteries.
Consideration was given to many means of power generation and power storage. This included primary chemical batteries of all sorts, nuclear batteries, rechargeable batteries, and the separation of the stimulator system into two parts, with the power pack being outside the patient's body and transmitting pulses of energy to a passive implanted receiver and lead. Cardiac pacemakers based on rechargeable nickel-cadmium systems and rechargeable zinc-mercuric oxide systems were developed. Such pacemakers are described in prior art references, including U.S. Pat. Nos. 3,454,012; 3,824,129; 3,867,950; 3,888,260; and 4,014,346. The rechargeable pacemaker incorporated a charging circuit which was energized by electromagnetic induction, or other means. A replaceable battery has also been proposed by Kraska, eta/, U.S. Pat. No. 4,010,760.
Because the power supply or battery has generally been the limiting component in both implantable pacemakers and implantable defibrillators, it would be advantageous to provide a implantable cardiac stimulator which has a replaceable battery. Thus, instead of replacing the entire stimulator, only the battery need to be replaced as the power source nears its ends of life. Alternatively, a faulty or obsolete electronic circuit could be replaced or a circuit having different features could be substituted for the prior circuit. This should provide substantial economic benefits for the patient.